Hi,
like stay on the download site of PLCSim (http://support.automation.siemens.com/WW/view/en/78793685?Datakey=47071380):

"

Remarks for PLCSIM:

If a version of the TIA Portal is already installed on your computer, PLCSIM V13 may
not be accessible when using STEP 7 Professional (for example, download).
Remedy: Start the PLCSIM V13 setup again and use the repair option.

"

It hapened with me, and the work around worked.